Story
Grisilla said that the Poison Blood had to be purified directly at the Root of the Sacred Tree. Assist Grisilla in purifying the Root of the Sacred Tree.
After completion (spoilers)
"I see... Grisilla returned to the Sacred Tree." Grisilla regained her strength at the Healing Spring after collapsing while trying to fight the Poison Blood alone. Still weak, she expressed her gratitude and said the Root of the Sacred Tree needed to be purified if the Forest's problems were to be solved once and for all. You and Grisilla purified the Poison Blood that had been inserted deep into the root of the Sacred Tree. Grisilla gave you Life Essence as thanks and returned to the Sacred Tree.